The present invention relates to a non acoustic alarm device, intended to be fitted to a portable unit in contact with the body, such as a watch, a portable telephone or a pager.
In numerous situations, it is useful to be able to transmit a piece of information to a person other than by acoustic or visual means. This is the case in particular when one wishes to alert a person who is in the middle of a meeting.
Tactile data transmission means offer an advantageous alternative for this purpose: a unit worn by the person next to the body, such as a watch for example, is made to vibrate, in order to stimulate his skin locally to indicate to him a given time or the occurrence of an event (arrival of a message, a call, a meeting, etc.).
Vibrating devices of the type with an unbalanced mass mounted on a rotor are known to those skilled in the art. In these devices, typically, the unbalanced mass rotates at a speed of several tens of revolutions per second thanks to an electric motor which is supplied with a power of several tens of milliwatts and started at the moment when the message has to be perceived by the wearer.
These devices have the main drawback of consuming a lot of energy, which is incompatible with the battery miniaturisation requirements encountered within the horological field.
European Patent Application EP 0 625 738 in the name of the Applicant discloses a vibrating device for a unit such as a watch. This device includes a coil electromagnetically coupled to a moving mass.
This patent application does not describe the characteristics of the excitation means of the coil. Having said this, those skilled in the art know that pulses whose frequency is equal to the natural oscillation frequency of the coil-moving mass unit have to be applied to the coil to obtain a maximum vibration amplitude for a given quantity of supplied energy.
In practice, this natural frequency is difficult to determine strictly. First, it varies from one coil-moving mass unit to another because of manufacturing tolerances, which are of the order of 15%.
Next, it varies as a function of the way in which this unit is worn, and forms a body with the wearer. Typically, the wear conditions induce variations of the order of 5% of the unit""s natural frequency, as well as a variation in the energy dissipated in the wearer.
These variations reduce the yield of the coil excitation means which are designed to operate at a fixed frequency, and this results in a significant waste of energy.
The object of the present invention is to overcome these drawbacks.
This object of the invention, in addition to others which will appear upon reading the following description, is achieved with a non acoustic alarm device, intended to be fitted to a portable device worn next to the body, such as a timepiece or a portable telephone, including a case, a mass moving within this case intended to transmit vibrations thereto, a coil electromagnetically coupled to said moving mass to make it vibrate, and an excitation circuit of said coil, this device being characterised in that it also includes means for measuring the instantaneous oscillation frequency of the coil-moving mass unit during the oscillation in progress, as well as means for generating a series of excitation pulses for said coil during the next oscillation, whose characteristics are a function of the measurement of said instantaneous oscillation frequency.
As a result of these characteristics, it becomes possible to make a non acoustic alarm device which does not require much energy and which operates with a good yield, which means that the use thereof in particular in timepieces of small dimensions, such as watches, can be envisaged.